Electrical steel sheet varnishes to coat electrical steel and individual steel sheets are known. The coated electrical steel sheets can be assembled together by different technical means such as welding, clamping, interlocking, aluminium die casting or riveting to form a solid core for the use in electrical equipment, such as, transformers, generators and motors. The coatings provide electrical insulation between the metal sheets in core and should be able to meet the requirements of high surface insulation resistance, resistance to mechanical stress and corrosion and thermal stability.
JP 2000345360 and EP-A 923 088 relate to enamels for coating electrical steel sheets wherein the enamels contain particles, such as, silica or alumina colloid particles. The compositions result in coatings having properties, such as, good scratch, blocking, chemical and corrosion resistance and high surface insulation ability. Such coatings have no bonding function and need additional means of bonding (welding, clamping, interlocking, aluminium die casting or riveting) to form a solid core.
There are some known coating systems in use for the coating of electrical steel sheets suitable for, e.g., welding or punching applications to form a solid core. In view of this, the core sheet varnish selection is frequently a compromise since there are occasions when a single coating will not fulfil all requirements. The known ranking classes of such coatings, for example class C3, class C5, class C6 (registered as standards under AISI-ASTM A 976-03) show the different requirements of coatings in this field with regard to such properties. The coating may be only an organic mixture (C3 insulation type) or an organic/inorganic mixture of complex resins and chromate, phosphate and oxides (C5 and C6 insulation type).
C3 coatings based on organic resins, e.g., phenol, alkyd, acrylic and epoxy resin are used. The C3 coating will enhance punchability and is resistant to normal operating temperatures but will not withstand stress-relief annealing. In general, the C5 coating can be on the one hand a semi organic coating with very good punchability and good welding response and on the other hand a basically inorganic coating with organic resins and inorganic fillers, which has excellent welding and heat-resistance properties with good punchability. But C5 coatings are generally based on chromate-, phosphate- or on titanate-compounds, and they are therefore not environmental friendly, particularly with respect to the remaining carcinogenic level, or they can tend to hygroscopicity and insufficient annealing and corrosion resistance, or can show insufficient welding properties. C6 coatings are organic coatings with a high content of fillers approximately of 50 wt %.
The known systems are not able to combine different technical requirements, such as, welding, clamping, interlocking, punching, riveting, pressure resistance and thermal resistance to provide a high property profile standard.
EP-A 2152792 and EP-A 2222805 disclose varnishes for coating individual electrical steel sheets. The varnishes can comprise self-crosslinkable binder resins as co-binders which can be epoxy novolak systems and urethane and acryl modified epoxy resins. Such varnishes often do not have an acceptable solid content for a given viscosity of the compositions.
Known are self-crosslinkable systems based on transesterification as a crosslinking reaction for the manufacture of coatings for magnet wires. See for example DE1445263, DE1494452, DE1645435, EP616628 or U.S. Pat. No. 4,446,300. Such coatings do not meet the properties necessary for electrical steel, for example, because the wettability and the solid content/viscosity ratio is insufficient and because they are typically dissolved in phenol, cresol and other organic solvents which do not fulfil the strict environmental requirements that apply for standard core sheet varnishes.
Accordingly, it is desirable to provide coating processes for electrical steel sheets to provide a high property profile standard for welding, clamping, interlocking, punching, riveting, high pressure and thermal resistance. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.